Chimps find human yawn contagious, but is it monkey see, monkey do?

A young chimpanzee will yawn when a human does, regardless of whether the face is familiar, according to a study that suggests contagious yawning grows stronger in our primate cousins as they grow up.

Researchers yawned, gaped or wiped their nose in front of 33 chimpanzees orphaned by Central Africa's illegal bush meat trade and housed at a Sierra Leone primate rehabilitation center. They expected the chimps would more readily copy their "mother," a villager who feeds, cuddles and grooms the captive chimps, some of which arrived when they were a few months old.

Instead, infant chimps didn't respond to the human gestures. The older juveniles, however, "caught" the contagion, yawning readily at their adoptive mother and researchers alike, according to a study published online Wednesday in the journal PLOS One.

The study was the first to demonstrate the contagion across primate species, according to its lead investigator and face-maker, Elainie Alenkaer Madsen, an evolutionary psychologist at Sweden's Lund University.

While you're stifling that contagious yawn, stifle your instinct to assume the chimps are just like us. Madsen, who also has made faces at puppies, says the primates aren’t showing the cognitive empathy exhibited by humans, who can readily put themselves in someone else's shoes.

“I think we’re talking more about affective empathy,” Madsen said. “It’s the kind of empathy where, instead of thinking your way into how someone else might be experiencing the world or feeling, you just feel it. Like when someone cuts their finger you feel sick in your stomach.”

Even among humans, the theory of mind required to infer someone else’s mental state doesn’t appear until about 4 years of age. And research has yet to demonstrate that other primates ever develop a theory of mind.

Still, contagious yawning is widespread among primates such as chimpanzees, baboons and bonobos. Even dogs do it.

A study in 2011 found that adult chimpanzees tended to yawn more readily in response to members of their own group, so Madsen expected that facial familiarity would make a difference in yawn contagion among the rescued Sierra Leone chimps.

“We’re more likely to empathize with those we’re familiar with, so we really expected the adoptive mother of the chimps would be able to provoke more contagious yawning, with the younger chimps in particular,” she said. “But there was no such thing, which is puzzling.”

“Maybe chimps just respond differently to humans than they would to other chimps,” Madsen suggested. Previous yawn contagion studies only examined contagion among adult chimps, usually through video images. The traumatized Sierra Leone chimpanzees could be a bit more likely to associate any human face with affection, fun and help, she said.

Dogs between 4 and 14 months old likewise don't discriminate in their yawning response, Madsen found. Another study this year found that adult dogs did discriminate.

“It’s possible that it’s a developmental pattern across the species,” Madsen said.

So, why do we yawn contagiously? Mimicry has a neural basis in mirror neurons, found in primates and some birds. Neuroscientists believe these copycat nerves lie at the root of the various forms of unconscious imitation.

While humans have abundant mirror neurons that fire in response to stimuli, some brain imaging studies have shown that mirror neuron areas don’t activate during contagious yawning. But yawning did activate areas associated with social skills and processes that require self-referential processing.

If there is some evolutionary reason why contagious yawning would be shared among primates, brain temperature could offer a clue. Studies suggest that yawning helps regulate brain temperature, including the mild site-specific increases associated with sleepiness. Patients with brain disorders associated with poor temperature regulation also display atypical yawning patterns, studies have shown.

For social animals, a collective yawn could be nature's way of making sure everyone is on the same page -- shaking off drowsiness or settling in for the night.

When Madsen yawned for puppies, they tended to settle down and sleep. And she wasn't immune from the so-called behavior transfer.